There are two passages in the New Testament that deal specifically with apostasy and in each passage it is treated with the gravest concern and dire warnings. In both cases it leaves little hope for anyone who would turn from grace to works to achieve salvation.
The first can be found in Galatians and the writer of Galatians, Paul, is more than a little upset that the Galatians would be so easily misled by the Judaizers that insisted the Galatians had to add circumcision to the finished work of Christ. "Behold I, Paul, say to you that if you receive circumcision, Christ will be of no benefit to you. And I testify again to every man who receives circumcision, that he is under obligation to keep the whole law. You have been severed from Christ, you who are seeking to be justified by law; you have fallen from grace." There is only one other instance when Paul makes this claim about the possibility of losing the life granted by the Spirit. That is in Hebrews.
This was a battle Paul had to fight many times in his ministry. He found it very difficult to convince many of the Jewish believers that the covenant of the Mosaic Law was done away with when Christ paid the penalty of sin on the cross and made a new covenant written in His blood. The old strictures of the law no longer applied. Grace and the love of God had changed everything. The Kingdom of God accepted everyone, Jew or Gentile, on the basis of faith in the Jewish Messiah, Jesus of Nazareth. God had redeemed His creation and that meant all men, not just Jews. The only merit available to man was the blood of Jesus and that was apprehended by faith. It ended the sacrifice of animals, and all the ritual and dietary laws that had separated the Jews from the Gentiles. Live and walk by the Spirit and not the flesh. The Spirit is life and peace, the flesh is sin and death. Trying to establish righteousness by fleshly obedience to law presupposed a loss of faith in the grace of God.
In Hebrews we see a somewhat different situation but the essence of the problem is the same. At the time Hebrews was written some Jewish believers were returning to Judaism to escape the persecution initiated by Nero. Paul does not attach his name to the epistle because many of the Jewish believers disliked him intensely and would not have read the letter if he had written it. Hebrews is one of many letters that Paul wrote that is a treatise without equal in Christian writing. The writer masterfully expounds Christ as the perfect sacrifice, prophesied in the Old Testament. Proclaimed by angels. Established by signs and wonders. A great high Priest after the order of Melchizedek. In chapter two he warns, "How shall we escape if we neglect so great a salvation?" He warns them not to harden their hearts reminding them of those who fell in the wilderness and never entered the promised land. He explains that the sufferings of Christ make Him an intercessor who understands their suffering.
At the end of chapter four we read, "Let us therefore draw near with confidence to the throne of grace, that we may receive mercy and may find grace to help in time of need." In chapter six we come to that famous passage that many use to prove that it is possible to fall from grace. Beginning at verse 4, "For in the case of those who have once been enlightened and have tasted the of the heavenly gift and have been made partakers of the Holy Spirit , and have tasted the good word of God and the powers of the age to come, and then have fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame."
For Paul the greatest evil was to forsake the Covenant made in Christ blood and turn from faith in the grace of God back to the slavery of the Law. Christ had brought freedom from the tyranny of the law and freedom from bondage to sin. He understood the enormity of what the Jews had done in not accepting Jesus as messiah. He could rejoice in the Salvation that He had in Christ and desired with all his heart the same thing for his people, the nation of Israel. He understood better than most the futility of the old covenant and its endless sacrifices and rituals. All the meaning attached to them no longer meant anything. It was indeed death for those that remained.
No doubt Paul felt that this kind of apostasy was the worst. To return to a futile religion when one had the opportunity to worship God in Spirit and in truth. How could anyone contemplate giving up being a Spirit filled child of the King with the promise of eternal life in His presence. How could they know this and return to trying in vain to establish a righteousness they never had in the first place. I dare say it was almost more than he could comprehend. For himself it was unthinkable and he used every argument he could muster to convince them of the gravity of the error. He does leave them with hope, "But, beloved we are convinced of better things concerning you, and things that accompany salvation, though we are speaking in this way." Towards the end of chapter six we read these words, "It is impossible for God to lie, we may have strong encouragement, we who have fled for refuge in laying hold of the hope set before us. This hope we have as an anchor of the soul, a hope both sure and steadfast..."
These two examples make it clear to me that if it is possible to lose salvation it is not by any sin but the by a loss of faith in Christ and the redemption He bought for us and exchanging that for a dead religion that sets out to establish a righteousness of the flesh that is in no way acceptable to God by any means. It is in the attempt to add to or take away from the finished work of Jesus Christ our Lord and Savior.